“1971” is an appropriately matter-of-fact title for a decidedly low-key documentary. But don’t mistake a lack of flash for an absence of substance. The story told here couldn’t be more significant or more timely.

As directed by Johanna Hamilton (who co-produced the excellent “Pray the Devil Back to Hell”), “1971” tells all about a half-forgotten clandestine event that took place on the night of March 8 of that year, an incident that had a more enduring impact than even its participants anticipated.

While much of America was watching the long-awaited heavyweight championship fight between Muhammad Ali and Joe Frazier, a group of men and women broke into an FBI office in Media, Penn., a small town near Philadelphia, and walked out with every file in the place, more than 1,000 documents all told.

This group, which called itself the Citizens’ Commission to Investigate the FBI, did more than take the documents, they sent the most incriminating to members of Congress and key media outlets. Only the Washington Post allowed its reporter, Betty Medsger, to publish the contents of what was unabashedly stolen material, and the results were incendiary.

For what those Media documents revealed was the first proof of the extent of government snooping on, and intimidation of, ordinary citizens, revelations that led to the first congressional investigation of intelligence organizations.

More than that, those documents led, a year later, to a Freedom of Information search initiated by NBC News’ Carl Stern that revealed how the bureau threatened and coerced activists like Martin Luther King Jr. through a shocking program called COINTELPRO.